Guitar players are often fickle with competing requirements for their amplifiers. On the one hand they want simplicity while on the other hand they need versatility. They revere vintage amplifiers for their “classic purity,” but then lament the lack of modem features. They demand a certain type and model of amplifier in order to duplicate influential recordings but are then frustrated by the typical limitations of these early designs, the most common being the “one-good-sound-at-one-volume” syndrome. And that's usually too loud!
The fundamental limitation of these vintage amplifiers is that tone is inseparably related to volume. Many of the vintage amplifiers popularized in the 1960s by famous bands such as the Beatles produce preferred tones only when set at or near maximum volume. While such settings are acceptable for many live venues, they are too loud to be useful in a typical home or small club setting. When adjusted to lower volume, the tone suffers as the overdrive, which alters the tone, evaporates completely.
One such vintage amplifier is the Vox AC-30 and its sibling the AC-15. These amplifiers are revered for their unique performance and simple appearance. As testimony to their lasting impact, a modernized version has been introduced by a firm resurrecting the Vox brand name. Furthermore, several copies and derivatives of the original design that have been produced by other manufacturers. But all of these suffer from either being versatile but complex or being faithful to the original but limited in functionality.
Even though the Vox AC-15 and AC-30 amplifiers are rated at a mere 15 or 30 watts respectively, they are surprisingly loud. Moreover, their iconic tones really don't materialize until they are set near the top of the output range via the one simple Volume control
This limitation has been dealt with successfully in the past with other circuits, typically by the inclusion of separate controls to regulate preamp output amplitude independent from preamp gain. In many amplifiers one or more extra stages of amplification are added as well. U.S. Pat. No. 4,211,893 (“the '893 patent”) of Randall C. Smith presented one such solution by allowing Gain and Loudness to be preset independently and providing two different foot-switchable sounds—a clean one for Rhythm and an overdriven one for Lead. This solution is adequate for many players but necessitates four separate controls, it is a clear departure from the simplicity of an AC type amplifier with its single Volume control. Furthermore, this design may be inadequate for some players and musical styles because the Rhythm mode of the '893 design may have insufficient gain while the Lead mode has too much.
Despite the bare-bones simplicity of the Vox circuit and control panel, the range of gain available from the single Volume control is impressive. Clean sounds at lower settings give way to semi-broken-up sounds to moderately overdriven sounds when the single Volume control is turned up to its maximum. While this range of gain is adequate for many popular styles of music, the most desired sounds only occur at very loud levels, severely limiting the Vox's usefulness especially in smaller venues.
Compounding the problem is that a typical Master attenuator control, coming at the end of the preamp, will not work with the Vox AC type amplifier circuitry. Their original preamp circuit is far too deficient in gain, comprised of only a single triode gain stage ahead of the phase splitter and power tubes. Later Vox AC models added Bass and Treble controls plus a second triode to recover amplitude lost through those tone controls. Even so, there is still not nearly enough gain to obtain usable overdrive characteristics from the preamp alone.
The Vox AC amplifiers compensated for this lack of preamp gain by employing a long-tail driver and push-pull output stage and operating them “open loop,” with no negative feedback. The cathode coupled, long-tail phase splitter uses two separate triodes to furnish push-pull drive signals and contributes substantial gain in the process. What's more, it is this “wide open” output section that is clearly fundamental to the unique character and success of the Vox's musical performance.
Another unique aspect of the Vox was the inclusion of a so-called “Cut control” to partially compensate for the initial lack of tone controls and to allow some taming of the brash sounding wide-open output section. This consisted of a capacitor and variable resistor functioning as a high pass filter between the input grids of the push-pull output tubes. Advancing the Cut control clockwise increasingly attenuates high frequencies by decreasing the resistance between the opposing grids such that the high frequencies were selectively cancelled through the capacitor.